1. Field of the Invention
The present invention is directed to filter cartridges that employ track-etched membranes as their filter media, and methods of making such filter cartridges.
2. Discussion of the Related Art
Filter cartridges are used to remove impurities from fluids in a number of different applications, such as water purification, chemical processes wherein impurities are removed from reaction mixtures during product synthesis, and medical applications wherein impurities are filtered from a liquid before the liquid is passed into a patient. Filter cartridges conventionally include a filter medium formed from a sheet of porous material that is pleated and folded into a corrugated cylinder to provide a large filtration area in a relatively small volume. The cylinder of filter material is inserted into a housing that provides structural rigidity to the filter cartridge, and the housing is then closed at each end with an end cap. The untreated fluid is directed through the sidewalls of the housing and passed through the filter, which removes fine impurities and contaminants from the fluid. The filtered fluid flows into a passageway in the center of the cylindrical filter material, and is removed from the cartridge through an outlet port in one or both of the endcaps.
Filter cartridges are typically required to provide a predefined level of purification by preventing any impurities above a specified size from passing therethrough. To maintain the integrity of the cartridge and ensure that no untreated fluid can pass therethrough, the filter medium is sealed in a fluid-tight manner to the endcaps, such that all fluid exiting the output port of the cartridge is guaranteed to have passed through the filter medium.
The filter media used in conventional filter cartridges are typically porous sponge cast membranes that include a number of cavities that are designed to produce the effective pore size required for the filter cartridge. Such membranes suffer from several disadvantages. First, the arrangement of cavities is designed to provide a predetermined effective pore size only to a statistical degree of certainty, and cannot guarantee that paths do not exist through the membrane that exceed the effective pore size, such that contaminants could potentially slip through the filter medium. Second, because of its construction, the sponge cast member absorbs some material that is smaller than the effective pore size of the filter and should be passed through, thereby undesirably altering the filtrate. Third, the particles filtered from the fluid cannot be recovered from the sponge cast membrane, which is disadvantageous for some applications.
Track etched membranes have been used as filters in a number of applications, and do not suffer from the above-described disadvantages of sponge cast membranes. Track etched membranes are formed by bombarding a solid film with particles that form tracks of damaged material through the film. The film is then subjected to a chemical agent that selectively etches the damaged tracks to create perforations through the film. The diameters of the perforations can be controlled by the residence time of the etchant on the film. Thus, the film can be provided with pores that all are equal to or smaller than the maximum pore size required for the filtration application, ensuring that no impurities can pass through the film that are larger than the specified maximum pore size. Furthermore, the particles filtered from the liquid can be easily recovered from the track etched membrane, which also does not trap material that is smaller than the membrane pores.
In view of the foregoing, it has been desired to provide a filter cartridge that includes a track etched membrane as the filter medium. However, conventional methods for forming filter cartridges have been found to be ineffective in sealing a track etched membrane to the cartridge end caps, resulting in faulty seals and cartridges that fail integrity testing. For example, U.S. Pat. No. 3,457,339 discloses a method for sealing a conventional filter medium to a pair of filter cartridge end caps formed from thermoplastic material. The end caps are initially heated to a temperature above the softening point of the thermoplastic material, and the filter medium is then inserted downward into the softened end cap and imbedded therein. The end cap is then allowed to cool. The thermoplastic material is said to penetrate the pores of the filter material to ensure a leak-proof seal.
Although the technique described in the '339 patent has apparently been successful in connection with other types of filter media, it cannot be used with a track etched membrane. In particular, the assignee of the present application has unsuccessfully attempted to produce a filter cartridge using substantially similar techniques for sealing a track etched membrane to the end caps. The failed sealing technique involved heating each end cap until it was molten, or providing a mass of molten material in the end cap, and then inserting the end of the track etched membrane downward into the molten end cap. The assembly was allowed to cool, in the hope that the molten material would form a fluid-tight seal between the end cap and the track etched membrane. However, integrity provable cartridges could not be made using this sealing technique, as gaps in the seal between the filter membrane and the end caps resulted in impurities bypassing the membrane and contaminating the filtrate. It is believed that the failure of this conventional sealing technique is attributable to track etched membranes being significantly thinner than conventional filter media. Thus, it is believed that when the track etched membrane was inserted downward into the molten end cap, the heat radiating therefrom caused the thin membrane to wilt upward, resulting in gaps between the resulting end cap seal and the track etched membrane.
Another technique for sealing a filter medium in a filter cartridge is disclosed in U.S. Pat. No. 4,956,089. In accordance with this technique, a die is provided in the shape of the end caps to be formed, and includes a heater disposed underneath. The filter medium and several support layers, which are formed from thermoplastic material, are inserted into the heated die and melted to form a layer of molten material that fills the recesses of the die. The material is then allowed to cool, forming an end cap with the filter medium imbedded therein. Because this technique also involves the insertion of the filter medium downward toward a heated element, it is believed that this technique would also fail to successfully seal a cartridge including a track etched filter membrane for the same reasons as the above-described failed technique.
Another conventional technique for sealing a filter medium to an end cap is disclosed in U.S. Pat. No. 4,392,958. In the disclosed method, an adhesive is spread onto the end caps, and the filter medium is inserted into the adhesive, which seals the filter medium to the end cap. A significant drawback to this technique is that in the filter cartridges formed thereby, the adhesive can leach into the solution, resulting in solvent contamination. Therefore, filter cartridges formed in accordance with this method often do not provide the desired level of filtration, limiting their utility.
In view of the foregoing, it is an object of the present invention to provide an integrity testable leak-proof filter cartridge that includes a track etched membrane, and a method for its formation.